TODO deprecated releng namespace

The "releng" namespace is flagged as deprecated over an alternative
"releng_tool" namespace. This is being done for the following reasons:

1) The namespace "releng_tool" definition is more unique namespace for the
   releng-tool project. This prevents possible collisions with a namespace
   "releng" which has a much higher possibility of being used by end-developers
   (e.g. a group/developer may have its own "releng" namespace of scripts to
   deal with their own group/project-specific release engineering use cases;
   which could conflict with releng-tool, if integration is desired).

2) Provide flexibility for adding the `.py` extension to all project-defined
   scripts. The original release of releng-tool defined a project-specific
   configuration script `releng.py` which now is named `releng`. This change was
   primarily driven for users invoking releng-tool using the module name where
   the releng-tool entrypoint is not available (i.e. invoking `python -m releng`
   would load the local script over the installed releng-tool module).

At this time, this project still also uses the "releng" namespace for a window
to help ease transition of existing releng-tool-attached scripts and extensions
which may be using the "releng" namespace.
